Corner reflector



Dec. 15, 1959 o. HALPERN 2,917,739

CORN

Unite States Patent 2,917,739 CORNER REFLECTOR Otto Halpern, Pacific Palisades, Calif., assignor, by mesn'e assignments, to the United States of America as represented by the Secretary of the Navy This invention relates to an identification device and more particularly to a reflector for reflecting radio microwave radiation having a surface which is periodically made reflectingand non-reflecting of incident microwave radiation.

-,In the operation of radar equipment it is frequently desirable to be able to identify a target appearing upon the screen of the indicator scope. This invention provides a novel means for identifying a radio microwave reflector by modulating reflected signals from the reflector in such a manner as to provide an automatic recognition device which is operative at preselected wave lengths of incident microwave radiation.

An objectof this invention isto provide an identification device comprising a radio microwave reflector having means for automatically modulating radio microwave radiation impinging upon the reflector.

Another object of this invention is to provide a radio microwave reflector having means responsive to a preselected wave length of incident radio microwave radiation for modulating the signals reflected by the reflector.

Still another object of this invention is to provide a radio microwave reflector having one surface which is periodically made reflecting and non-reflecting of incident radio microwave radiation.

A particular object of this invention is to provide a radio microwave reflector having means responsive to a preselected wave length of incident radio microwave radiation for causing one surface of the reflector to be periodically reflecting and non-reflecting of such incident radio microwave radiation.

Further objects and advantages of this invention, as well as its construction, arrangement, and operation will be apparent from the following description and claims in connection with the accompanying drawings, in which:

Fig. 1 is a plan view of the reflector as contemplated by this invention; and

Fig. 2 is a front view of a reflector as shown in Fig. 1.

It is known that radar signals (radio microwave radiation) reflected by moving objects show intensity modulation which has characteristic and more or less constant frequencies. For example, radar signals reflected from a three-bladed propeller show a modulation which has a frequency equal to three (number of blades) times the propeller shaft frequency and its harmonics. This invention provides an identification device comprising a radio microwave reflector having means for automatically modulating radio microwave radiation reflected by the reflector.

Referring 'to Figs. 1 and 2, there is shown a metallic trihedral corner reflector 20 having two fixed intersecting triangular surfaces 21 and 22 which preferably include an angle of 60 therebetween in the vertical plane and an angle of 90 therebetween in the horizontal plane. The third side of the reflector consists of a sector of a rotating disc 23 which is rotated about the axis of a shaft 24 by means of a suitable motor 25. Surfaces 21 and comprise a pair of identical triangularly shaped sur- 2,917,739 Patented Dec. 15, 1959 faces which intersect along the edge oppositely disposed from disc 23, thereby forming a corner reflector having only. a single open end 29 for admission and reflection of incident radio microwave radiation. As shown in the drawings the plane of the open end '29 is normal to the plane of the disc 23. A yoke 26 is provided tomaintain surfaces 21 and 22 in a fixed orientation with respect to motor 25, and at the same time to permit disc 23 to rotate with respect to surfaces 21 and 22.

In order to modulate the reflected microwave radiation, disc 23 is provided in certain sectors with a coating of material which is non-reflective of incident radio microwave radiation. In the particular design shown, two opposite quadrants 27 and 28 are covered with the nonreflecting material while the remaining opposite quadrants 30 and 31 are left reflective and the corner reflector will thus modulate reflected signals with a fre-- quency equal to twice that of the rotational speed of the disc; That is to say, in a reflector constructed as shown, the radar signals reflected by the reflector will show a modulation which has a frequency equal to two times the disc shaft frequency. It will of course be readily understood that disc 23, comprising the third surface of the corner reflector and being related, will be alternately reflective and non-reflective of incident microwave radiation of the wavelengthfor which the coating is made resonant. 7

Various materials may be used to coat portions of disc 23, among them being Harp material which is responsive only to a preselected wave length or band of wave lengths for whichit is designed to be resonant. Harp material is composed of myriads of finely divided electrically conductiveparticles dispersed quasi-insulated in a neutral binder. Such finely divided particles preferably include aluminum, graphite, copper, and permalloy flakes, although other electrically conductive particles may be used. Examples of such binders include waxes, resins, polystyrene vistanex, and synthetic rubbers among others. Such a material is fully disclosed and described in my copending application Serial Number 581,179, filed March 5, 1945. The characteristic property of Harp material is a high dielectric constant, and in the case where ferromagnetic flakes are used, a high magnetic susceptibility, properties which give a high index of refraction for electro-magnetic waves. The physical principle underlying these applications is the construction of a' coating of material having a high index of refraction and a thickness equal to a quarter-wave length or odd multiple there- I i of, as measured inside the coating, of the incident microwave radiation for which it is designed to be resonant. Such a coating, being a resonant device, has a reflection coefficient which increases rapidly on both sides of the resonant wave length. However, there is very little reflection of incident microwave radiation of the wave length for which the coating is designed to be resonant. It is therefore possible to obtain an arbitrarily sharp resonance effect which is selective to a particular microwave frequency and wave length.

As shown in Fig. l, quadrants 27 and 28 are covered with a coating of Harp material as heretofore described, the thickness of the layer being equal to a quarter-wave length, or odd multiple thereof, as measured inside the coating, of the incident microwave radiation for which it is designed to be resonant. In this manner, an electrical dissymmetry which is effective only at the particular radiation frequency for which the coating is made resonant is introduced between the coated and uncoated portions of disc 23. Incident microwave radiation of the resonant frequency of the coating, being reflected by corner reflector 20, is modulated in accordance with rotation of disc 23, while microwave radiation of other fre modulation signals other than that equal to twice the ro-.

tational speed of the disc as explained above. While a particular embodiment of this invention has been disclosed and described, it is to be understood that various changes and modifications may be made in this invention without departing from the spirit and scope thereof as setforth in the appended claims.

What is claimed is:

1, An identification device comprising a corner reflector for reflecting incident radio microwave radiation, said reflector having a plurality of inclined surfaces, one of said surfaces being rotatable with respect to the remainder of. said surfaces and having sectors thereof non-reflective of incident radio microwave radiation ofa predetermined wave length.

2. An identification device comprising a corner reflector having a plurality of inclined metallic surfaces for reflecting incident radio microwave radiation, .one of said surfaces being rotatable with respect to the remainder of .said surfaces and having various sectors thereof coated with a layer for preventing reflection of incident radio microwave radiation of a preselected frequency therefrom, said layer consisting of myriads of finely-divided electrically conductive particles dispersed quasi-insulated, in a binder, the thickness of said layer being predeterminedly limited to anodd multiple of one-quart r Wave length of the incident microwave radiation of preselected frequency, as measured inside the layer.

3, A reflector comprising a plurality of reflecting surfaces, at least one of said surfaces being rotatable with respect to the remainder of said surfaces and having selected sectors thereof absorptive of radiation of a preselected wave length.

4. A reflector comprising a plurality of reflecting surfaces, at least a portion of one of said reflecting surfaces having a coating of material absorptive of radiation of a predetermined wave length.

5. A reflector comprising a plurality of reflecting surfaces, at least one of said surfaces being rotatable with respect to the remainder of said surfaces and having first selected sectors absorptive of radiation of preselected wave lengths and second selected sectors reflective of radiation of said preselected wave lengths.

6. A reflector comprising a plurality of reflecting surfaces, at least one of, said surfaces being rotatable with respect to the remainder of said surfaces and having preselected successive;se.ctors alternately reflective and absorptive of radiation of preselected wave lengths.

References Cited in the file of this patent UNITED STATES PATENTS 4 1,384,014 Fessenden July 5, 1921 2,212,110 Beuermann Aug. 20, 1-940 2,432,984 Budenbom Dec. 23, 1947 2,443,643 Schelleng June 22, 1948 2,472,782 Albersheim June 14, i949 FOREIGN PATENTS 422,659 Great Britain Jan. 16, 1935 694,523 Germany July 4, 1940 OTHER REFERENCES The Corner Reflector Antenna, Proceedings of ;I. R LE., vol. 28, No. 11, November 19.40, pp. 513 519.

The Square-Comer Reflector, Radio for March 1939, pp. 19-23. 

